This application claims the priority benefit of Taiwan application serial no. 90101241, filed Jan. 19, 2001.
1. Field of Invention
The present invention relates to light emitting device. More particularly, the present invention relates to a structure of light emitting diode having a superlattices contact layer. The light emitting diode is a nitride-base III-N group compound semiconductor device.
2. Description of Related Art
In recent years, gallium nitride-based III-N group compound semiconductor device, such as GaN, GaAlN, and GaInN, has been greatly taken as a light emitting device. FIG. 1 is a cross-sectional view, schematically illustrating structure of a conventional light emitting diode made of III-N group compound.
In FIG. 1, the light emitting diode is formed on a substrate 10, such as an Al2O3 substrate. A nucleation layer 12 and an N-type conductive buffer layer 14 are sequentially formed over the substrate 10. The buffer layer 14 includes GaN doped with N-type dopant, so as to ease the crystal growth for the subsequent crystal growing process. There is a light-emitting active layer 18 over the buffer layer 14. Usually, the active layer 18 is confined by a confinement layer, that is, cladding layers 16, 20. The cladding layers 16, 20 are doped with opposite conductive type. For example, if the lower cladding layer 16 is the GaN layer doped with N-type dopants, the upper cladding layer 20 is the GaN layer doped with P-type dopants. Then, a contact layer 22 is formed on the upper cladding layer 20. The contact layer 22 a P-type GaN layer. A transparent electrode layer 24 is formed on the contact layer 22, where the transparent electrode layer usually includes a N-type material layer, such as indium tin oxide (ITO), cadmium tin oxide (CTO), or ultra-thin metal. The transparent electrode serves as an anode of the diode. Moreover, an electrode layer 26, serving as a cathode of the diode, is also formed on the buffer layer 14 but is separated from the cladding layers 16, 20 and the active layer 18.
FIG. 2 is a cross-sectional views, schematically illustrating a light emitting region for the light emitting diode in FIG. 1. In FIG. 2, when the electrodes 24, 26 are applied with a forward bias, the diode is conducted. At this situation, current can flow from the electrode 24 to the active layer 18. In the conventional manner, the P-type contact layer 22 of GaN cannot have high carrier concentration and has large contact resistance. This results in a poor quality of current spreading. The p-type electrode layer 24 also only covers a portion of the contact layer 22. As shown in FIG. 2, the area having current flow is about the width L of the electrode layer 24. This limits the light emitting area for the diode. The function of the active layer cannot be fully performed. The light emitting efficiency of the diode is then greatly reduced.
In summary, the conventional light emitting diode is restricted by the physical properties of the contact layer. The contact layer cannot be grown with a high hole concentration. This also causes the high fabrication cost and also causes low yield. Further still, the conventional structure cannot provide a diode with high light emitting efficiency. A large portion of the active layer 18 of the diode is not well utilized. Moreover, the conductive doping types for the electrode layer and the contact layer are different. It could cause a junction between them at the contact region, and affecting the operation of the diode.
The invention provides a structure of light emitting diode, which has a contact layer having structure of doped strained-layer supperlatices (SLS), so that the contact layer can easily have a high carrier concentration, resulting in high conductivity.
The invention provides a structure of light emitting diode, which uses strained-layer supperlatices structure to serve as a contact layer associating with the transparent electrode layer, so as to improve the light emitting efficiency and reduce the operation voltage.
The invention provides a structure of light emitting diode, which has a contact layer having structure of doped strained layer supperlatices (SLS). The dopant type in the contact layer is therefore not necessary to be restricted. The transparent electrode and the contact layer can even be the same conductive type, so that the junction between the transparent electrode and the contact layer is avoided.
The invention provides a stricture of light emitting diode, which has a contact layer having structure of doped strained-layer supperlatices (SLS). The transparent electrode has better contact quality. The area of the transparent electrode can be about equal o the area of the active layer. This can allow the current to flow through the larger area of the active layer, so that the effective light emitting area is increased, and the light emitting efficiency is accordingly increased.
The invention provides a structure of light emitting diode, which is formed on a substrate, including:
a nucleation layer and a buffer layer with a first conductive type are sequentially formed on the substrate. A lower cladding layer with the first conductive type is formed on the conductive buffer layer, wherein the doped materials in the first-type cladding layer and the conductive buffer layer have the same conductive type, such as P-type or N-type. An active layer is located on the first-type cladding layer to serve as a light emitting layer for the light emitting diode. An upper cladding layer with a second conductive type is located on the active layer. The first-type cladding layer and the second-type cladding layer have doped materials with opposite conductive type. A second-type contact layer is located on the second-type cladding layer. The contact layer includes semiconductor material with periodic variation and modulated doped, such as doped Mg, Zn, Be, Cd, CA, C, or Hg, all of which has a structure of strained-layer superlattices. A transparent electrode is located on the contact later to serve as an anode. Another electrode layer, serving as a cathode, contacts with the conductive buffer layer, and is separated from the lower and upper cladding layers, the active layer, the contact layer and the transparent electrode.
In the foregoing, the transparent electrode layer and the SLS layer can have different conductive types. The transparent electrode layer can have the conductive type of P-type or N-type.
The invention provides a structure of light emitting diode, having an SLS structure formed on the substrate. A conductive buffer layer is formed on the substrate, so as to ease the crystal growth for the subsequent process. The active layer is located over the lower cladding layer. The active layer includes doped III-N group compound semiconductors. The upper cladding layer is located on the active layer. The SLS layer is located on the upper cladding layer. The conductive type of the contact layer can be different from the upper cladding layer. A transparent electrode layer is located on the SLS layer. Another electrode layer has a contact with the conductive buffer layer but is separated from the active layer and the transparent electrode layer.
In the foregoing, the transparent electrode layer and the SLS layer can be the same conductive type with all P-type or all N-type. The transparent electrode layer and the SLS layer can also be different conductive type.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.